1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly to an LCD with improved image retention for increased reliability and screen quality.
2. Description of the Prior Art
A fringe field switching mode (hereinafter, referred to as FFS-mode) LCD has been proposed to improve the low aperture ratio and transmittance of an in-plane switching mode (hereinafter, referred to as IPS-mode) LCD, as disclosed in Korean Patent Application No. 98-9243.
The FFS-mode LCD has counter and pixel electrodes made of a transparent conductor for higher aperture ratio and transmittance than the IPS-mode LCD. Furthermore, the gap between the counter and pixel electrodes is smaller than that between upper and lower substrates to form a fringe field between the counter and pixel electrodes. As a result, all liquid crystal molecules are actuated, including those positioned on top of the electrodes, for improved transmittance.
A conventional FFS-mode LCD will now be described with reference to FIG. 1, which is a sectional view thereof.
As shown in FIG. 1, a black matrix layer 2 is formed on an upper substrate 1 to interrupt transmission of light and a color filter layer 3 including red, green, and blue is formed thereon. An over-coating layer 4 is formed on the black matrix layer 2 and the color filter layer 3. A common electrode 12 is formed on a lower substrate 11 in a plate shape, and a gate insulation layer 13 is formed on the resulting substrate including the common electrode 12. A data line 14 is formed on the gate insulation layer 13, and a protective layer 15 is formed on the resulting substrate including the data line 14. A pixel electrode 16 is formed on the protective layer 15 in a slit shape. Alignment layers 5 and 17 are formed between the upper substrate 1 and a liquid crystal layer 18 and between the lower substrate 11 and the liquid crystal layer 18, respectively, to determine the initial arrangement of liquid crystals.
The conventional FFS-mode LCD, however, has no electrode formed on the upper substrate, in contrast to a TN-mode LCD. Since the black matrix layer and the over-coating layer are made of a material having very high specific resistance, furthermore, image retention is very likely to occur. Particularly, the FFS-mode LCD has a larger capacitor than the TN-mode LCD. The resulting adsorption of impurity ions and the influence of the remaining DC components generate severe image retention.
An alignment agent for forming the alignment layers includes a polymer compound and a solvent for dissolving the polymer compound to improve coating characteristics. The polymer compound is used to align the liquid crystals in a predetermined direction. The solvent is a liquid-state material having low viscosity for easy application of the polymer compound and is used to retain the alignment agent. The alignment agent, including the polymer compound and the solvent, has large specific resistance during application and increases the capacitance inside the LCD. This results in the image retention of the LCD.